Our studies are directed at seeking serological evidence of recent or current viral infection and/or allergy in victims of the Sudden Infant Death Syndrome (SIDS). We are doing this by measuring the levels of serum and secretory immunoglobulins in these children and testing their serum and bronchial washings for various anti-viral antibodies of the IgM and IgA classes using an indirect radioimmunoassay. To date we have collected material on 24 cases of infant death, 15 of which fit the usual definition of SIDS. The only difference we have thus far observed between SIDS victims and infants: a) dying of well defined causes, or b) dying under conditions where accidents, gross neglect or child abuse seem to be contributing factors, is that more of the SIDS victims (7/10 tested thus far) have detectable levels of IgE greater than 25 units/ml in serum and/or greater than 5 units/ml in bronchial washings) than the controls (1/5). None of the infants tested have clearly pathological levels of IgE, however, the highest value observed to date being 65 units/ml of serum.